1. Field of the Invention
The present invention relates to a constant-speed cruise control device for a vehicle, and more particularly, to a constant-speed cruise control device for a vehicle also provided with a fuel cutoff control device to stop fuel supply to an internal combustion engine in a case where a fuel cutoff condition has been obtained.
2. Related Art
Conventionally, a constant-speed cruise control device for a vehicle is a device which, when a vehicle-speed setting switch for constant-speed cruising use has been operated after a driver has depressed an accelerator pedal and accelerated to a desired vehicle speed, performs control so that the vehicle speed at that time is established as a vehicle speed for constant-speed cruising use and thereafter causes a difference between vehicle speed and established vehicle speed to be zero, for example regulating a throttle degree of opening to cause the vehicle to cruise automatically at a uniform speed. A vehicle mounted with this device is extremely convenient in that there normally exists no need to depress the accelerator pedal in a case of uniform-speed travel on an expressway or the like.
When a vehicle mounted with the constant-speed cruise control device as described above is cruising at constant speed at a previously set target vehicle speed, the foregoing constant-speed cruise control device performs control so as to close the throttle valve in a case where vehicle speed becomes larger than target vehicle speed due to gravitational acceleration, for example, at entry into a steep descending slope. Moreover, the throttle valve is closed to a fully closed position in a case where the difference does not easily become zero.
However, in a case where the vehicle is mounted with a fuel cutoff control device, if an idle switch goes on while during the foregoing full closure of the throttle, a fuel cutoff condition is established and fuel is no longer supplied to the internal combustion engine, and so output torque of the internal combustion engine drops sharply and vehicle speed also drops sharply. In a case where vehicle speed becomes smaller than target vehicle speed, the foregoing constant-speed cruise control device opens the throttle valve and attempts to increase speed. In this way, the idle switch goes off, fuel cutoff control stops, and fuel is supplied, and so the output torque of the internal combustion engine rises sharply and vehicle speed also rises sharply. If vehicle speed rises and exceeds the target vehicle speed, the throttle valve is again closed and the idle switch goes on, whereupon fuel cutoff is started and vehicle speed drops sharply. This phenomenon is repeated thereafter as long as descending slopes continue. That is, as shown in FIGS. 12A-C, vehicle speed comes to change in small steps up to the timing from X to Y.
This vehicle-speed fluctuation is termed fuel cut/surge, and is physically an extremely unpleasant phenomenon.